A known motion control device for a vehicle is disclosed in JP3385812B. According to the motion control device disclosed, in order to achieve an appropriate advancing vehicle speed when the vehicle advances to the curve, a deceleration distance required for the vehicle to be decelerated for advancing to the curve and a remaining distance defined from the current position of the vehicle to a curve starting point are obtained. Then, a speed reduction control is started when the remaining distance that is decreasing matches the deceleration distance. The speed reduction control is finished when the remaining distance is equal to zero and the vehicle passes over the curve starting point.
In addition, another motion control device for a vehicle is disclosed in JP2005-170152A. According to the device disclosed, in order to reduce the uncomfortable feeling given to a driver by the acceleration failure in the vicinity of a curve ending point, which results from the speed reduction control when the vehicle is driven through the curve, the control variable of the speed reduction control is decreased when it is determined that the vehicle is traveling before the curve ending point, thereby reducing the deceleration of the vehicle is reduced. In other words, the speed reduction control is not finished until the vehicle passes over the curve ending point.
As illustrated in FIG. 16, generally on a road, a single curve is constituted by an advance transition curve zone (i.e., transition curve zone on the entrance side), a constant curvature radius zone, and an exit transition curve zone (i.e., transition curve zone on the exit side) in this order towards a curve ending point (curve exit) from a curve starting point (curve entrance). The transition curve is, for example, structured by a clothoid curve. The transition curve zone is provided so that the vehicle smoothly travels through the curve while the driver gradually steers the steering wheel and then returns the steering wheel without an abrupt operation thereof.
When assuming the long advance transition curve zone, a start and/or an end of the speed reduction control is conducted relatively earlier than the intention of the driver in a state where the speed reduction control is performed so that the speed reduction is finished at the curve starting point as in the device disclosed in JP3385812B, thereby giving the driver the uncomfortable feeling.
Further, the uncomfortable feeling of the driver occurs when the vehicle is decelerated by the control even though the driver attempts to accelerate the vehicle, the vehicle cannot be accelerated because of the control even though the driver desires to accelerate, and the like. In particular, the driver tends to look at a point a certain distance ahead of the vehicle and to predict the vehicle condition at that point. That is, the driver does not look immediately ahead of the vehicle. Thus, in a case where the speed reduction control is not finished until the vehicle passes over the curve ending point as disclosed in JP2005-170152A, the uncomfortable feeling of the driver may remain.
A need thus exists for a motion control device for a vehicle which is not susceptible to the drawback mentioned above.
Further, still another motion control device for a vehicle is disclosed in JP2004-142686A. According to the device disclosed, a shape of a curve existing ahead of the vehicle on a road on which the vehicle is traveling is obtained. The speed reduction control for decelerating the vehicle is performed independently from the deceleration operation by the driver.
More specifically, one appropriate vehicle speed is specified for the vehicle traveling through the curve. In a case where the vehicle speed is greater than the appropriate vehicle speed when the vehicle advances to the curve, the speed reduction control is started at a predetermined position. In the speed reduction control, first, the deceleration of the vehicle is controlled by using a first target deceleration. Afterwards, the vehicle deceleration is controlled by using a second target deceleration, which is greater than the first deceleration, instead of the first target deceleration so that the vehicle is decelerated to the appropriate vehicle speed. Accordingly, when the speed reduction control is started in a state where the driver does not recognize the presence of the curve ahead of the vehicles the uncomfortable feeling unexpectedly given to the driver by the start of the speed reduction control, and the like may be reduced.
Following two cases are considered for the requirement of the speed reduction control at a time of the vehicle advancing to the curve. First, in a case where the deceleration operation is performed by the driver, the speed reduction control is required to assist the driver's deceleration operation by decelerating the vehicle to the appropriate vehicle speed. In this case, the appropriate vehicle speed is desirably specified to be sufficiently smaller than a vehicle speed for a turning limit of the vehicle. The speed reduction control in such case will be hereinafter referred to as a travel assistance control.
Second, in a case where the driver does not recognize the curve existing ahead of the vehicle, where the driver wrongly recognizes a radius of the curve to be larger than the actual radius, and the like, the speed reduction control is required to ensure the traveling stability of the vehicle by decelerating the vehicle to the appropriate vehicle speed so that a turning level (state) of the vehicle does not exceed the turning limit thereof. In this case, the speed reduction control is performed regardless of whether or not the deceleration operation is performed by the driver. At this time, different from the travel assistance control, the appropriate vehicle speed is desirably specified adjacent to a vehicle speed for the turning limit of the vehicle. The speed reduction control in such case will be hereinafter referred to as a limit protection control.
According to the device disclosed in JP2004-142686A, only one appropriate vehicle speed is specified for the vehicle traveling the curve. That is, the aforementioned two types of speed reduction control are not assumed or considered and therefore are not selectable depending on the operation condition of the vehicle.
A need thus exists for a motion control device for a vehicle, in order to address the drawback mentioned above, which can effectively perform a speed reduction control by selecting and using the aforementioned two types of speed reduction control depending on the traveling state of the vehicle.